We speak of a functional unit of cells that work together to create and release a product in a duct or directly into the bloodstream.
There are two main types of glands, exocrine and endocrine. There is an important distinction between the exocrine and endocrine glands.
While the exocrine glands secrete substances in a ductal system to an epithelial surface, the endocrine glands secrete products directly into the bloodstream.
Exocrine secretions are formed in the acinus, a small group of cells at the origin of the glandular ducts. Exocrine glands are sub-classified into subtypes based on the method of secretion, the compound produced, or the shape of the gland.
The three mechanisms by which the exocrine glands release their secretions include merocrine, apocrine, and holocrine. The merocrine glands are the most common subtype. By definition, the secretions of the merocrine glands leave the cell through exocytosis.
In this secretion method, there is no cell damage. An example of merocrine secretion is the eccrine sweat gland. Apocrine glands, by contrast, form membrane buds that rupture in the duct, losing part of the cell membrane in the process.
A known apocrine gland is the breast milk-producing mammary gland. The final subtype of excretion is holocrine, in which the cell membrane ruptures to release its product into the duct. The sebaceous glands are a representation of the holocrine secretion.
The exocrine glands are made up of an acinus and a duct with different types of cells, respectively. These glands are found in many organs within the body and show a great variety in the function of their secretions.
As such, there is a wide range of cell types in the exocrine glands.
While the duct functions primarily to transport glandular secretions, the acinus is responsible for the production of glandular secretions and as such shows more variety in cellular composition.
Typical cell types within the acinus include serous, mucinous, or sebaceous. The serous cells secrete an isotonic fluid that contains proteins such as enzymes.
The salivary glands are largely made up of serous cells. The mucinous glands secrete mucus. A typical example is Brunner’s glands in the duodenum.
The sebaceous glands secrete sebum, an oily compound. Oil glands are most common on the face, scalp, groin, and armpits.
Cell types can also be differentiated histologically. Mucous cells generally stain lighter than their serous counterparts when stained with hematoxylin and eosin.
As the ducts move from the acini to the end goal, secretions initially enter the intralobular duct. Intralobular ducts have a simple cuboidal epithelium, commonly surrounded by parenchyma.
Intralobar ducts drain into interlobar ducts which are a simple columnar epithelium.
The final ductal unit is the interlobar duct recognized by a stratified columnar epithelium. Connective tissue surrounds the interlobar and interlobar ducts.
The initial manifestation of exocrine gland formation is epithelial budding resulting from a complex interaction between mesenchymal and epithelial cell populations.
This initial period of ingrowth is influenced by fibroblast growth factors, especially FGF10 and cadherin-2. Other transcription factors that have been shown to contribute to epithelial sprouting include HlxB9, Isl1, LEF-1, Msx1 / 2, Pbx1, Pdx1, and Tbx3.
After the initial formation of the epithelial bud, ductal lengthening occurs. This process is mediated by a large group of molecular signals such as Netrin-1, TIMP1, amphiregulin, IGF1, and leukemia inhibitory factor.
Several matrix metalloproteinases (MMPs) contribute to basement membrane renewal and facilitate ductal elongation.
After an initial period of ductal lengthening, the exocrine gland begins to form ductal branches. NF-kappa-B is believed to play a role, as well as sonic hedgehog and Wnts.
As the duct begins to elongate, the acinus undergoes a period of cell proliferation and differentiation. Due to the great variety in the function of the exocrine gland, the exact number of cellular signals and interactions is immense.
In general, however, there is an important role for cell adhesion molecules such as laminin and cadherins.
Exocrine morphogenesis is a rapid process. Ductal lengthening and branching usually occur in less than a week, with acini formation 5 to 9 days later.
In a relatively short period of development, exocrine glands form and can begin to secrete a functional product.
Organ systems involved
Due to the diverse number and function of epithelial surfaces in the body, exocrine glands are used by many organ systems to carry out their respective actions. Several examples will be included here including skin, mouth, stomach, pancreas, duodenum, and sinuses.
The skin has a variety of exocrine glands, including eccrine sweat glands and sebaceous glands. The eccrine sweat glands are the most widespread sweat glands in the body and are present on almost all external surfaces of the body.
The sweat produced is clear with little or no oil, which unlike the sebaceous glands, which are also found in the skin, which secrete the oilier substance of sebum.
The salivary glands in the mouth are another example of exocrine glands and include the parotid glands, the submandibular glands, and the sublingual glands.
While each gland has a unique mix of serous and mucosal cells, together the salivary glands act to begin the process of digesting food while at the same time lubricating and protecting mucosal surfaces.
The stomach contains multiple forms of exocrine glands including pyloric glands, heart glands, and fundic glands. These glands incorporate many different types of cells, including parietal cells, main cells, and G cells.
Together they regulate gastric pH, release enzymes to break down food products into a digestible form, and help with the absorption of necessary vitamins and minerals.
The pancreas has an endocrine function and an exocrine function. The exocrine pancreas aids in the digestion of food by releasing a secretion rich in bicarbonate, which helps neutralize the acidic environment created in the stomach. The secretion also includes digestive enzymes.
Brunner’s glands are present in the duodenum of the small intestine. These exocrine glands are submucosal and produce a mucous product that protects the duodenum from acid released by the stomach.
The alkaline nature of the secretion also activates intestinal enzymes to help with the breakdown and absorption of food.
The mammary gland is one of the best known examples of an exocrine gland found in the breast. The mammary glands produce nutrient-rich milk that also provides passive immunity to the baby’s immune system.
The specific function of the exocrine glands within the body varies depending on the location and organ system. However, the main function is to create a secretion that is subsequently released through a ductal system on an epithelial surface.
Examples include secretions that help in:
- Digestion of food.
- Protection of the mucosa.
- The nutrition.
In general, tests are not performed for a large individual exocrine function. However, dysfunction of the exocrine glands can create a wide range of clinical manifestations.
Imaging can be done to confirm a diagnosis in cases of blocked glands. Sialolithiasis refers to cases where a stone lodges within the salivary gland or duct, and sialadenitis refers to inflammation of the gland.
CT and ultrasound are effective methods for identifying and locating stones.
The liver itself acts as an exocrine gland by creating and excreting bile for storage in the gallbladder awaiting expulsion and release through the pancreatic duct into the duodenum.
Obstruction at any point in this pathway can cause cholecystitis, due to inflammation and dysfunction of the gallbladder.
Ultrasound is the initial diagnostic test to diagnose cholecystitis.
In cystic fibrosis, sodium and chloride are not reabsorbed into the sweat duct due to a dysfunctional CFTR protein, resulting in abnormally salty skin. The sweat chloride test is the primary test for the diagnosis of cystic fibrosis.
Pancreatic insufficiency occurs when the exocrine glands of the pancreas can no longer produce the digestive enzymes necessary for the breakdown of food in the small intestine.
Common etiologies include chronic pancreatitis, cystic fibrosis, and hereditary hemochromatosis.
Several methods can be used to assess the function of the exocrine pancreas. Fat malabsorption can lead to deficiencies in fat-soluble vitamins A, D, E, and K. Therefore, vitamin levels can be used to estimate pancreatic function.
The fecal elastase-1 test is another method with relatively high specificity and sensitivity. Low levels of fecal elastase-1 indicate a malfunctioning exocrine pancreas.
However, the most sensitive diagnostic method for exocrine pancreatic insufficiency is to use direct tests of pancreatic function such as the cholecystokinin (CCK) or the secretin stimulation test.
Sjogren’s syndrome is commonly associated with rheumatoid arthritis and other rheumatic diseases. The syndrome is an autoimmune disorder that demonstrates a decreased function of the lacrimal and salivary glands that may also have associated systemic symptoms.
The disease is characterized by dry eyes and mouth due to gland dysfunction. Due to dry mouth, patients with Sjogren’s syndrome show higher rates of oral candidiasis and dental caries.
Cystic fibrosis is an autosomal recessive disease that causes impaired chloride transport due to a mutation of the CFTR protein.
Because CFTR is involved in the production of sweat, mucus, and digestive fluids, the mutation has a direct effect on exocrine gland secretions.
In fact, about 90% of babies born with cystic fibrosis will develop pancreatic insufficiency by one year of age.
The prevalence of acne is estimated at 35 to 90% in adolescents. The disorder affects the pilosebaceous unit, of which the sebaceous glands are an example.
The pathogenesis is multifactorial and often involves follicle hyperkeratinization, increased sebum production, and proliferation of Propionibacterium acnes with associated inflammation. As sebum builds up, an open comedo forms, also known as a white head.
Hyperkeratinization and increased sebum production lead to clogging of the pores of the pilosebaceous unit. As the lipids within the sebum oxidize, the follicular orifice opens, forming an open comedo or pimple.
Treatment for acne is highly dependent on the severity of the inflammatory symptoms, but topical retinoids are often the first-line treatment, although antimicrobial agents are an additional option for refractory cases.
For severe cases of nodulocystic acne or for patients who have failed treatment with systemic antibiotics, oral isotretinoin is the therapeutic option.
The exocrine gland can be found in many organs and serves a wide variety of functions within the body. Due to this fact, an understanding of the physiology of the exocrine glands is essential for healthcare workers.
Exocrine glands can be found in everything from the skin to the pancreas, and they provide the body with a method of releasing secretions containing proteins, mucus, and other products to epithelial surfaces around the body.
Exocrine gland dysfunction is associated with diseases as broad as acne vulgaris to Sjogren’s syndrome.